Turbo decoder with extrinsic information scaling modules

ABSTRACT

The invention related to a turbo decoder comprising SISO decoding modules each other interconnected in a feedback control scheme having scaling modules for applying a scaling factor to extrinsic information delivered by said SISO decoding modules. The turbo decoder comprises a selection module for adaptively selecting said scaling factor based on a number of decoding iterations of the turbo decoder.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a national filing in the U.S. Patent &Trademark Office of PCT/IB2007/052266 filed Jun, 14, 2007, and claimspriority of EPO Patent Application No. 06300727.2 filed Jun. 27, 2006,both of which applications are incorporated herein in their entiretiesby this reference.

The present invention relates to turbo decoders, more specifically to aturbo decoder implementing iterative control algorithm for suppressingtransients.

Turbo encoding/decoding techniques are nowadays widely used in the fieldof telecommunication, including GSM communication, communication withspatial probes, and so on.

Several turbo decoding techniques have been designed, one of them beingthe so-called “next iteration initialization turbo decoding technique”which uses an control algorithm for enhancing the transient behavior ofthe turbo decoding. Such technique is described by D. Spasov et al. in“A practical algorithm for Turbo decoding enhancement”, proceedings ofthe 2004 IEEE

International Symposium on Circuits and Systems, vol. IV, pp. 621-624,2004, ISBN 0-7803-8251-X.

The turbo decoder described in this paper comprises two usualinterconnected SISO (Soft Input Soft Output) decoding modulesimplementing a log-map or max-log-map algorithm. Each decoding modulesis included in an feedback control loop provided for the other module,this loop being designed to suppress transient chaos appearing in theso-called “water-fall region”.

The feedback control loop comprises a scaling module connected to eachdecoding module and applying a scaling factor to extrinsic informationreceived from it. In Spasov's paper, the scaling factor depends on somecharacteristics of the extrinsic information, e.g. on some constantparameters as well as on an exponential of the absolute value of theextrinsic information. The computation of the scaling factor thus addscomplexity to the overall system and is time consuming.

The present invention aims at solving the aforementioned problem andprovides a turbo decoder comprising at least two SISO decoding moduleseach other interconnected in a feedback control scheme having scalingmodules for applying a scaling factor to extrinsic information deliveredby said SISO decoding modules, wherein it comprises a selection modulefor selecting said scaling factor based on a number of decodingiterations of the turbo decoder.

According specific embodiments of the invention:

-   -   said selection module implements a decreasing function of said        number of decoding iterations for selecting said scaling factor;    -   said selection module implements a linear function;    -   said scaling factor is comprised between 0.5 and 1.0; and    -   the scaling factor is selected amongst a set of pre-computed        values.

The invention also provides a turbo decoding method for decoding turbocodewords, comprising at least two SISO decoding steps interrelated in afeedback control scheme and a scaling step for applying a scaling factorto extrinsic information computed during said at least two decodingsteps, wherein it comprises a selection step for adaptively selectingsaid scaling factor based on a number of decoding iterations of theturbo decoding method.

The invention, as well as advantages thereof, will be better understoodwhen reading the following description, given by way of example and inconnection with accompanying drawing in which:

FIG. 1 is a schematic diagram of a turbo decoder according theinvention;

FIG. 2 illustrates a law of selection of scaling factor value infunction of iteration number used by a selection module of FIG. 1; and

FIG. 3 is plots of bit error rate vs. signal/noise ratio of a turbodecoder of the prior art and a turbo decoder according the invention.

A turbo decoder 10 of the invention is schematically represented at FIG.1.

The turbo decoder 10 receives turbo codewords (p, p1, P2) at inputterminals 12, 14, 16 respectively, the turbo codewords being transmittedto turbo decoder 10 over AWGN (additive white Gaussian noise) binaryinput memoryless channel (non represented), using BPSK (binary phaseshift keying) modulation. A turbo codeword is composed of first block ofpayload data p, a second block of n/2 parity binary bits p1 for thepayload data computed using convolution code and third block of n/2parity binary bits p2 for a known permutation of the payload data, as itis well known in the art. The bit length of a turbo codeword is thusequal to 3n, with e.g. n=1000.

The turbo decoder 10 comprises a first and second conventional SISOdecoders 18, 20.

The first decoder 18 is connected to input terminals 12, 14 forreceiving the payload data p and the n/2 parity binary bits p1. Thefirst decoder 18 also receives at input terminal 22 scaledde-interleaved extrinsic information data sded2 from the second decoder20, as it will be described in greater details hereafter, and providesat an output terminals 26, 28 extrinsic information data ed1 and decodeddata dd1 respectively.

Likewise, the second convention SISO decoder 20 is connected to inputterminal 12 through a first interleaver 28 and to the input terminal 16for receiving interleaved payload data ip and the n/2 parity bits p2.The second decoder 20 also receives at input terminal 30 scaledinterleaved extrinsic information data sied1 from the first decoder 18,as it will be described in greater details hereafter, and provides at anoutput terminals 32, 34 extrinsic information data ed2 and decoded datadd2 respectively. The first and second decoders 18, 20 implements aniterative Log-Map or Max-Log-Map algorithm, e.g. those described in theaforementioned Spasov's paper.

The turbo decoder 10 also comprises a decision unit 36 connected tooutput terminals 28, 34 for receiving decoded data dd1, dd2 from thefirst and second decoders 18, 20 and determining in function thereof thefinal decoded data dd, as it is well known in the art.

The first and second SISO decoders 18, 20 are interconnected in afeedback control loop scheme, one decoder being included in a feedbackcontrol loop for the other decoder.

In that way, a feedback control loop for the first decoder 18 comprisesa first multiplier 38 connected at an input thereof to extrinsicinformation data output terminal 24 of the first decoder 18, a secondinterleaver 40 connected at an input thereof to an output of multiplier38, the second decoder 20 connected at its input terminal 30 to theoutput of the interleaver 40, a second multiplier 42 connected at aninput thereof to extrinsic information data output terminal 32 of thesecond decoder 20, a de-interleaver 44 connected at an input thereof toan output of second multiplier 42. Finally, an output of thede-interleaver 44 is connected to the scaled de-interleaved extrinsicinformation data input terminal 24 of the first decoder 18, thus endingthe feedback control loop for the first decoder 18.

Likewise, a feedback control loop for the second decoder 20 is composedof the second multiplier 42, the de-interleaver 44, the first decoder18, the first multiplier 38 and the second interleaver 40.

The first and second multipliers 38, 42 multiply the extrinsicinformation data they receive by a scaling factor sf whose value isselected by a selection module 46 amongst a pre-computed set {sf1, sf2,. . . , sf10} of scaling factor values initially stored in a look-uptable 48.

The selection module 46 selects a decreasing scaling factor value sf asthe number of iterations of the decoding algorithm implemented by thedecoders 18, 20 is rising, thereby making the first and second decoders18, 20 more and more independent while said decoding algorithm isconverging.

Examples of selection functions implemented by the selection module 46are illustrated in FIG. 2 which is plots of selected scaling factorvalue in function of the number of iterations.

In a first embodiment, the selection module 46 implements a selection ofdecreasing scaling factor values comprised between 0.5 and 1.0 based ona decreasing function f1 comprising a first portion D1, which is astrictly decreasing linear function between the first iteration of thedecoding algorithm and a preset number of iterations it#10, and a secondportion D2, which is a constant function for number of iterationsgreater than it#10.

In a second embodiment, the selection module 46 implements a selectionof decreasing scaling factor values comprised between 0.5 and 1.0 basedon a decreasing function f2 which is an approximation of function f1. Inthis embodiment, the interval [0.5;1.0] of scaling factor values issampled in 11 values 0.5+i*0.05, where i=0, 1, 2, . . . , 10. Likewise,the interval [1; it#10] of numbers of iterations is sampled in 11 values1+i*(it#10−1)/10. The selection modules 46 thus selects the scalingfactor value 0.5 +i*0.05 if the number of iterations is comprisedbetween interval [1+i*(it#10−1)/10; 1+(i+1)*(it#10−1)/10] for any i<10and 1.0 for the scaling factor it the number of iterations is greaterthan it#10.

Other selections for the scaling factor are possible, e.g. selectionbased on function starting from a scaling factor equal to 1.0 andconverging to a scaling factor equal to 0.5 as the number of iterationis rising.

FIG. 3 illustrates the gain obtained by the turbo decoder of theinvention compared to the turbo decoder described in Spasov's paper.FIG. 3 plots the bit error rate (BER) vs. the signal/noise ratio (SNR)obtained by theses turbo-decoders. As one can see, by means of anadaptative scaling factor based on the number of decoding iterations,the BER of the turbo-decoder of the invention is lower than the oneobtained by the prior art, especially when the SNR is high.

From reading the present disclosure, other variations and modificationswill be apparent to the skilled person. Such variations andmodifications may involve equivalent and other features which arealready known in the art, and which may be used instead of, or inaddition to, features already described herein.

Although the appended claims are directed to particular combinations offeatures, it should be understood that the scope of the disclosure ofthe present invention also includes any novel feature or any novelcombination of features disclosed herein either explicitly or implicitlyor any generalisation thereof, whether or not it relates to the sameinvention as presently claimed in any claim and whether or not itmitigates any or all of the same technical problems as does the presentinvention.

Features which are described in the context of separate embodiments mayalso be provided in combination in a single embodiment. Conversely,various features which are, for brevity, described in the context of asingle embodiment, may also be provided separately or in any suitablesub-combination.

The applicant hereby gives notice that new claims may be formulated tosuch features and/or combinations of such features during theprosecution of the present application or of any further applicationderived therefrom.

For the sake of completeness it is also stated that the term“comprising” does not exclude other elements or steps, the term “a” or“an” does not exclude a plurality, and reference signs in the claimsshall not be construed as limiting the scope of the claims.

1. A turbo decoder having a hardware portion comprising SISO decodingmodules interconnected in a feedback control scheme having scalingmodules for applying a scaling factor to extrinsic information deliveredby said SISO decoding modules, and a selection module for adaptivelyselecting said scaling factor based on a number of decoding iterationsof the turbo decoder; wherein said selection module implements adecreasing function of said number of decoding iterations for selectingsaid scaling factor.
 2. The turbo decoder of claim 1, wherein saidselection module implements a linear function.
 3. The turbo decoder ofclaim 1, wherein said scaling factor is between 0.5 and 1.0.
 4. A turbodecoder of claim 1, wherein the scaling factor is selected amongst a setof pre-computed values.
 5. A turbo decoding method for decoding turbocodewords, comprising at least two SISO decoding steps interrelated in afeedback control scheme and a scaling step for applying a scaling factorto extrinsic information computed during said at least two decodingsteps, and a selection step for adaptively selecting said scaling factorbased on a number of decoding iterations of the turbo decoding method;wherein said selection step comprises a decreasing function of saidnumber Of decoding iterations for selecting said scaling factor.
 6. Themethod of claim 5, wherein said selection step comprises a linearfunction.
 7. The method of claim 5, wherein said scaling factor isbetween 0.5 and 1.0.
 8. The method of claim 5, wherein the scalingfactor is selected amongst a set of pre-computed values.
 9. A turbodecoder having a hardware portion comprising: a first decoder havingfirst and second inputs, an extrinsic input, an extrinsic output, and adecoded data output; a second decoder having first and second inputs, anextrinsic input, an extrinsic output, and a decoded data output; a firstinterleaver coupled between the first input of the first decoder and thefirst input of the second decoder; a second interleaver having an inputand an output coupled to the extrinsic input of the second decoder; afirst multiplier having a first input coupled to the extrinsic output ofthe first decoder, a second input for receiving a scale factor, and anoutput coupled to the input of the second interleaver; a secondmultiplier having a first input coupled to the extrinsic output of thesecond decoder, a second input for receiving the scale factor, and anoutput; and a de-interleaver having an input coupled to the output ofthe second multiplier, and an output coupled to the extrinsic input ofthe first decoder. a selection module for adaptively selecting saidscaling factor based on a number of decoding iterations of the turbodecoder, wherein said selection module implements a decreasing functionof said number of decoding iterations for selecting said scaling factor.10. The turbo decoder of claim 9 further comprising a decision unithaving a first input coupled to the decoded data output of the firstdecoder, a second input coupled to the decoded data output of the seconddecoder, and an output for supply a turbo decoder decoded data output.11. The turbo decoder of claim 9 wherein the first decoder comprises aSISO decoder.
 12. The turbo decoder of claim 9 wherein the seconddecoder comprises a SISO decoder.
 13. The turbo decoder of claim 9wherein the first input of the first decoder receives payloadinformation.
 14. The turbo decoder of claim 9 wherein the second inputof the first decoder receives parity information.
 15. The turbo decoderof the claim 9 wherein the second input of the second decoder receivesparity information.
 16. The turbo decoder of claim 9 further comprisinga lookup table for supplying the scale factor from a pre-computed set ofscaling factors.
 17. The turbo decoder of claim 16 further comprising amultiplexer coupled to the lookup table, the output of the multiplexersupplying a selected scale factor.
 18. The turbo decoder of claim 17wherein the multiplexer receives an iteration control signal.